1. Field of the Invention
The present invention relates to lighting engineering and, more specifically, to a light signalling device.
2. Description of the Prior Art
Readibility of light signal in the daytime, i.e. the ability of an observer to determine without error and during a short time the state of a signal light (e.g. whether the traffic light is on) depends both on the signal intensity and on the background illumination. External illumination causes flashes from the optical element, reduces the signal contrast or even produces false signals ("phantom effect"). The flashes may be seen in any direction. However, the phantom effect acting in the information-valued direction (along the self-radiation axis and in directions close to this axis) is especially harmful. To eliminate the phantom effect in stationary light signalling devices (traffic lights), sun visors are used, however, these have low efficiency in a low angular position of the sun and significally increase the overall dimensions of the device. In automobile signal lights, where the visors can hardly be used, the required contrast is obtained due to high light intensity. Stringent requirements to traffic safety necessitates higher intensity of the signal lights, which, however, cause glare by these lights at night. At the present time there are known optical systems, in which the phantom effect is eliminated using components, which are very expensive and can hardly mass-production motor cars.
Known in the art is a contrast light signalling device (GB, A, No. 1591013) comprising a concave reflector with a light source at the focal point thereof. In order to eliminate the phantom effect, the output aperture of the reflector is closed by a light absorbing screen having fine holes, through which the useful radiation is guided by means of lens rasters.
The idea of a light absorbing screen with light conducting holes is the basis of many later inventions (SU, A, No. 1173128). However, practical realization of this idea needs overcoming technological difficulties associated with making of perforated screens and rasters.
The concave reflector of a light signalling device may be either mirror-type or catadioptric, i.e. made of a transparent material in the form of prismatic elements featuring complete internal reflection (GB, A, No. 2069124). Both the catadioptric and mirror reflectors to be used in the light signalling devices require special arrangement against external illumination.
A device, which in its technical essence is close to the claimed solution, is a directional light signalling device--traffic light (Yu. A. Kremenets. M. P. Pechersky in the book "Technicheskie sredstva regulirovaniya dorozhnogo dvizhenia", 1981, Transport Publishers (Moscow), p. 77-81) comprising a convex paraboloidal reflector (collimator) with a light source at its focal point and an absorber of external illumination made in the form of black plates disposed in the meridian planes of the collimator ("antiphantom cross"). These plates have slots, which cannot be eliminated since a light source is located in these slots. External illumination rays pass through these slots near the collimiator focal point and, having been reflected by the paraboloidal mirror, are radiated along the collimator axis together with the useful rays of the device. Thus, while eliminating lateral flashes, this device is practically unprotected against external illumination in the most important information-values directions close to the axis of the light signalling device.